1. Field of the Invention
This invention relates to a position signal producing apparatus for producing a position signal indicative of position along an axis of an wafer on which an image on a reticle is to be projected, used in an exposure apparatus.
2. Description of the Prior Art
Recently, in semiconductor manufacturing apparatus, demand for high density arrangement of elements has been increasingly developed, so that thickness of circuit patterns of each element are reaching to a value lower than 0.5 .mu.m. In exposing processes of such fine circuit patterns, it is extremely important to consist one pattern position with another pattern at every exposing process to carry out many times of overwrite exposure which is necessary for producing semiconductor products. Such processing requires positioning accuracy of less than 0.1 .mu.m.
A position signal producing apparatus of a prior art is disclosed in Japanese patent provisional publication No. 63-78004. Hereinbelow will be described the above-mentioned prior art position signal producing apparatus with reference to FIG. 20.
FIG. 20 shows structure of the prior art exposure apparatus with the position signal producing apparatus. In FIG. 20, alignment light emitted from a laser light source 301, which is coherent and includes two frequency components. The alignment light is splitted into two light beams by a half mirror 302 and is diffracted by a first pair of diffraction gratings 304a and 304b formed on a reticle 303 respectively. Each diffracted light is projected onto a second pair of diffraction gratings 310a and 310b provided on a wafer 309 through a spatial filter 306a and 306b, a second lens system 307a and 307b, a projecting optical system 308. The diffracted light 311a and 311b is introduced into photodetectors 312a and 312b in the opposite direction through the projecting optical system 308 and the second lens systems 307a and 307b. When two beams are projected onto the second diffraction gratings 310a and 310b on the wafer 309 with suitable directions respectively, each diffraction light is diffracted in the direction such that diffracted light overlaps each other and interfere with each other. Light intensities of a pair of interfered diffracted light are detected by photodetectors 312a and 312b. The detected results are compared by a comparator 313. In accordance with the detected results, a control system 314 drives the wafer 309, so that difference of the above-mentioned diffracted light intensity equals zero. Thus, position aligning between the reticle 303 and wafer 309 is performed.
On the other hand, the pattern of the reticle 303 is illuminated by a projecting light source 315 and illumination optical system 316. The project image is focused onto the wafer 309 through the project lens 308.
However, the above-mentioned prior art structure is effective only when wavelengths of exposure light emitted from the project light source 315 and the alignment light emitted from the laser light source 301 substantially equal to each other and the project optical system 308 displays good focusing characteristic to both light. It is very difficult to build up an achromatic project optical system where ultraviolet light is used for exposure light, on the other hand visible light is used for alignment. This is because kinds of glass materials for making up a refracted optical system are limited at wavelength of ultraviolet light. For example, ultraviolet light is generated by a KrF excimer laser.
In other words, the project optical lens 308 is so designed as to sufficiently achromatize at exposure wavelength and thus, shows large chromatic aberration for other wavelength light. Therefore, it is desired that wavelength of the alignment light is sufficiently close to that of the exposure light. However, from point of view of producing semiconductor process, it is desired the wavelength of the alignment light is sufficiently separated from that of the exposure light. This is because a resist layer may be exposed by and respond to the alignment beam with a high sensitivity. For example, a chemically sensitized resist and a multilayer resist used for compensation of limit of focusing characteristic of the project optical system 308 may be used. Further, a resist layer containing die for preventing multi-reflection may absorb the alignment beam whose wavelength is close to that of the exposure light. In other words, wavelength of the alignment beam should be sufficiently separated from that of the exposure light from the reasons of semiconductor manufacturing process. Therefore, in the above-mentioned prior art structure, there is a problem that position alignment between the reticle and the wafer with high accuracy is difficult.